It is interesting to see in how far one or the other platform is used, and how the differences could be explained. The raw data are given below.

OMP pdf downloads 2016

The future of dialects

16137

Grammatical theory

7081

New directions in corpus-based translation studies

3798

Einführung in die grammatische Beschreibung des Deutschen

2981

The Alor-Pantar languages: History and typology

2546

Natural causes of language

2332

The empirical base of linguistics

2197

Roots of language

2066

Advances in the study of Siouan languages and linguistics

1795

Einführung in die grammatische Beschreibung des Deutschen²

1717

Linguistic variation, identity construction and cognition

1431

Thoughts on grammaticalization

1397

Adjective attribution

1291

The evolution of grounded spatial language

1216

A grammar of Yakkha

1192

Eyetracking and Applied Linguistics

1125

A grammar of Palula

1057

The Talking Heads experiment

941

Syntax und Valenz

933

The evolution of case grammar

889

Die Sprachwissenschaft

836

A grammar of Pite Saami

763

Language strategies for the domain of colour

729

How mobile robots can self-organise a vocabulary

712

A grammar of Mauwake

651

Prosodic detail in Neapolitan Italian

605

Grammaticalization in the North

568

A typology of marked-S languages

501

OAPEN pdf downloads 2016

A grammar of Pite Saami

2438

New directions in corpus-based translation studies

729

Roots of language

554

The empirical base of linguistics

527

Einführung in die grammatische Beschreibung des Deutschen

506

Linguistic variation, identity construction and cognition

275

Thoughts on grammaticalization

269

The Alor-Pantar languages: History and typology

245

Natural causes of language

222

Grammatical theory

208

The Talking Heads experiment

186

How mobile robots can self-organise a vocabulary

185

Prosodic detail in Neapolitan Italian

185

A typology of marked-S languages

180

A grammar of Mauwake

168

Advances in the study of Siouan languages and linguistics

167

The future of dialects

155

A grammar of Yakkha

150

Grammaticalization in the North

143

Die Sprachwissenschaft

140

The evolution of grounded spatial language

137

Language strategies for the domain of colour

135

The evolution of case grammar

103

Syntax und Valenz

92

A grammar of Palula

85

Adjective attribution

72

Einführung in die grammatische Beschreibung des Deutschen²101

69

Eyetracking and Applied Linguistics

7

It is obvious that the site langsci-press.org is much more frequented than OAPEN for our books. There are only a fraction of visits to our pdf documents at OAPEN as compared to the OMP site. One very interesting exception is A grammar of Pite Saami, which is by far the most popular book on OAPEN. It also fares well on the OMP platform, but is not within the top group.

A comparison of the download ranks is given below. Note that the long tail of OAPEN downloads is very close together and that the ranks do not really tell you much. Ranks #11 to #22 are separated by a mere 31 downloads at OAPEN (the difference is 668 at OMP).

Update:

Below are the figures for views on Google Books, and the comparison to OMP downloads. The top books correspond much better than in the OMP-OAPEN pairing.